Contaminants are often filtered from molten plastic before extrusion or injection. It is known, for example, to construct a filter which includes a rigid foraminous screen and an essentially tubular housing that supports the screen and direct a flow of molten plastic through it. The flow is n plastic at an upstream face of the screen. Heat is applied to the filter to maintain the molten state of the plastic during passage through the filter.
Clogging of the screen has been a problem. The screen is commonly mounted on a slide that permits it to be withdrawn from the housing for clearing. The screen is generally mounted in a circular seating structure. Upstream and downstream sides of the slide will commonly carry circular seals formed of bronze sealing material. The slide may have two screens that can be interchanged by sliding action. When one is in an operative position in-line with the flow of molten plastic, the other is normally external to the housing and available for cleaning.
The clogging problem is particular acute when processing recycled plastic scrap. Such scrap often contains significant quantities of metal debris, including large objects such as screws, bolts and nuts. In such an application, the filter tends to clog quickly. Also, given the very viscous nature of the plastic flow and the very high pressure required to drive the plastic through the filter, the screen can be very quickly damaged. It is also common for metal objects to be forced through the seals, leaving them severely scored and in need of replacement. The present invention address such problems.